With the present awareness of the importance of improving the efficiency of gas turbine systems in order to reduce fuel costs and also to reduce the emissions of greenhouse gases, there has been a continuing trend to operate such systems at higher turbine inlet temperatures. This results in combustor exit temperatures are exceedingly higher such that a limit is placed on achievable NOx emission levels at the required combustor residence times. Thus, essentially all gas turbine systems typically operating with a turbine inlet temperature greater than about 2200° F., known in the art as “F-Class” machines, even those with dry low NOx combustion systems, now require efficiency robbing after-treatment to achieve NOx emissions less than about 3 ppm. With emissions levels of 2 ppm or lower increasingly being required, there is a disconnect between the need for lower NOx emissions and the need for reduced emissions of greenhouse gases.
It is well known in the art that combined cycle gas turbine-steam turbine systems yield higher efficiencies than either simple cycle gas turbines or stand alone steam turbine power plants, particularly when such combined cycle systems operate with gas turbines exhaust gas temperatures of at least about 1000° F., more typically 1100° F. Use of higher efficiency gas turbines having pressure ratios much greater than 20/1 is usually disadvantageous without increases in turbine inlet temperature to maintain a sufficiently high exhaust temperature. There is therefore a need for gas turbine combustion systems capable of achieving ultra low NOx emissions at the higher required combustor outlet temperatures. Further, because the efficiency of typical high performance gas turbines is reduced at ambient temperatures below the design point, there is a need for a method to allow for improved efficiency at low ambient temperatures.
It has now been found that NOx emissions well below 3 ppm can be achieved even at combustion temperatures above those required for the highest inlet temperature gas turbines now available. By mixing cooled turbine exhaust gases with fresh air, inlet air comprising reduced oxygen content is supplied to the turbine compressor. Advantageously, the temperature of the air supplied to the compressor may be controlled to a predetermined value regardless of how low the ambient air temperature may be, thus allowing the turbine to be operated at maximum efficiency regardless of ambient conditions. If maximum power is required, the inlet air temperature can be reduced at some sacrifice in efficiency.